A nylon salt is herein understood to be a diammonium dicarboxylate salt, also indicated as a diamine/dicarboxylic acid salt, in other words a salt prepared from a diamine, or a mixture of diamines, and a dicarboxylic acid or a mixture of dicarboxylic acids. Polyamides prepared from such salts are generally indicated as AA-BB polyamides. Herein an AA-BB polyamide is understood to be a polyamide comprising alternating AA and BB repeating units, wherein AA represents the repeating units derived from diamine and BB represents the repeating units derived from dicarboxylic acid. Throughout this text the wordings “diammonium dicarboxylate salt” and “nylon salt” are meant to have the same meaning and are therefore interchangeable.
For the preparation of polyamides from nylon salts, different processes are known. The known processes include melt processes, solution processes and solid state processes. Known solid state processes include processes comprising solid state post condensation of polyamide prepolymer obtained by melt polymerization, solution polymerization followed by flashing and solid state post condensation and direct solid state polymerization processes. In such a process, the salt used is generally a granular material, such a powder, and the aim is also to obtain the resulting polymer as a granular material. The salt material used herein can be a salt powder or granular material obtained, for example, by spray drying, precipitation from solution, or a dry route process involving reaction of liquid diamine with solid dicarboxylic acid. The salt may also have a particular shape of compacted powder particles. Such a process is described, for example, in the U.S. Pat. No. 5,128,442 and GB-801,733.
In a melt process, a diamine, or a mixture of diamines, and a dicarboxylic acid or a mixture of dicarboxylic acids are mixed wherein at least one of the diamine or the dicarboxylic acid is in a melt form and the mixture of the two is obtained in a form of a melt. The resulting melt can be further polymerized to obtain a polyamide polymer.
Not all nylon salts are suited to be prepared in the melt, or even to be polymerized in the melt. In particular polyamides with high melting temperatures, such semi-crystalline semi-aromatic polyamides, tend to give side reactions when prepared in the melt.
In a solution process the diamine(s) and dicarboxylic acid(s) are first dissolved in a solvent. The solvent suitably consists of water or comprises a mixture of water and an organic co-solvent. The salt may be either isolated from the solution, and then polymerized by means of a suitable polymerization process, or first converted in the solution into a prepolymer and then isolated from the solution, and further polymerized by solid-state post condensation, or be converted into a prepolymer in the solution and then concentrated by evaporation of water and further polymerized in the melt.
Salts isolated from solution may be used for polymerization into polyamides. Known methods for the polymerization of isolated salts include melt polymerization, optionally followed by solid-state post condensation, and direct solid state polymerization.
For the isolation from solution different processes are known, including spray drying, crystallization induced by cooling followed by precipitation, precipitation induced by adding an organic non-solvent, or induced by evaporation of water and any co-solvent present.
Typically, the organic co-solvent as well as the organic non-solvent has to be miscible with water. An organic co-solvent and an organic non-solvent are herein understood to be different organic solvents, which differ by the fact that the nylon salt is soluble in the co-solvent, or in the mixture of water and co-solvent, whereas the nylon salt is not soluble in the non-solvent, or the mixture of the non-solvent and water.
Isolation by crystallization is an inherently slow process due to slow crystal growth rates, and is characterized by a very complex set-up of unit operations. Furthermore, co-crystallization of mixed salts, i.e. salts prepared from a mixture of diamines and a mixture of dicarboxylic acids is a challenge in itself.
Precipitation with an organic non-solvent has the disadvantage that relative large amounts of the organic non-solvent have to be used and that this non-solvent has to be recovered from water, resulting in large waste streams and a complex process as well.
Precipitation induced by evaporation of water and any co-solvent present can result in formation of big lumps, which can lead to difficulties in full removal of the solvent at the end of the process, and to problems with discharging the salt from the reactor, while big lumps need grinding and classification for further use in further processes.
Spray drying involves atomization of a solution and drying of small droplets in a counter-current flow of heated inert gas. The process typically results in a powder with relative small particles, and use of inert gas is essential to avoid the explosion risk which such particles pose in air. For the inert gas generally nitrogen gas is used. Spray drying is typically done in an apparatus comprising a very tall annular channel, or drying tower, equipped with multiple high pressure nozzles at the top of the channel, an inert gas stream, and a cyclone for separation of the fines from the inert gas stream. In a spray drying process, atomization of the solution into small droplets is critical, and also the droplet size needs careful control in order to obtain a dry powder at the bottom of the drying channel. For that purpose, nozzles with small dimensions and solutions with low viscosity have to be used. The solutions must be clear and free of impurities. The solutions must also be relatively highly diluted to relatively low concentrations, well below the saturation level of the salt, to prevent premature crystallization of the salt in the nozzles and to prevent blockage of the nozzles resulting from such premature crystallization. Due to the requirement of diluted solutions, large amounts of water need to be evaporated. This becomes even worse with nylon salts for semi-aromatic polyamides, which generally have a much lower solubility than salts for aliphatic polyamides. Almost all of the energy input needed for the evaporation has to come from the heated inert gas. This requires large volumes of heated gas to be circulated and heating with hot gas is inefficient and costly. Moreover, drying by heated inert gas requires large volume reactors and involves a complex process stream, since the inert gas needs filtering, heating, compressing and recycling,
Solution processes where nylon salts for semi-aromatic polyamides are prepared, comprising at least an aromatic diamine or an aromatic dicarboxylic acid, have the general disadvantage that the solubility of the salt is limited, and a large volume of water, or a mixture thereof with another solvent is needed.
Nylon salts can also be prepared in solid state under dry conditions, i.e. without the use of a solvent or a melt, by dosing liquid diamine to a solid powder of dicarboxylic acid under conditions that the dicarboxylic acid and the salt resulting thereof always remain below the melting temperature of the dicarboxylic acid and of the salt. A problem is that such a process is difficult to be carried out at large scale while applying conditions that the reactor content retains in a powdery state.
Therefore, in view of the above, there is need for a process for preparing a nylon salt that does not have the above disadvantages, or at least so in less extent, and in particular for a process suitable for preparing a semi-aromatic nylon salt in an effective and efficient manner.
The aim of the present invention is to provide a process for preparing a nylon salt in an effective and efficient manner.
This aim has been achieved with the process according to the invention, as mentioned in claim 1.
The process according to the invention is a flash process, wherein a hot, pressurized and concentrated aqueous mixture comprising a nylon salt dissolved or dispersed in an aqueous medium, is flashed at elevated temperature, upon which water is evaporated and a granular material is obtained.